Superalloy article having a gamma-prime nickel aluminide coating

Abstract

An article for use in hostile thermal environments, such as a component of a gas turbine engine. The article includes a nickel-base superalloy substrate that is prone to formation of a deleterious secondary reaction zone (SRZ), and an overlay coating having a predominantly gamma prime-phase nickel aluminide (Ni3Al) composition suitable for use as an environmental coating, including a bond coat for a thermal barrier coating. The coating comprises a chromium-containing nickel aluminide intermetallic overlay coating of predominantly the gamma prime phase, in which aluminum is present in the coating in an amount approximately equal to the aluminum content of the superalloy substrate so as to inhibit diffusion of aluminum from the overlay coating into the superalloy substrate.

Description

BACKGROUND OF THE INVENTION [0001] This invention relates to coatings of the type used to protect components exposed to high temperature environments, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to the combination of a nickel-base superalloy substrate prone to the formation of deleterious reactions with aluminum-containing coatings, and a predominantly gamma-prime (γ′) phase nickel aluminide overlay coating that reduces the incidence of such reactions when used as an environmental coating or as a bond coat on the superalloy substrate. [0002] Certain components of the turbine, combustor and augmentor sections that are susceptible to damage by oxidation and hot corrosion attack are typically protected by an environmental coating and optionally a thermal barrier coating (TBC), in which case the environmental coating is termed a bond coat that in combination with the TBC forms what may be termed a TBC system. Environment...

AI Technical Summary

Benefits of technology

[0008] The present invention generally provides the combination of a superalloy substrate and an overlay coating that environmentally protects the substrate when subjected to a hostile thermal environment, such as that found in the turbine, combustor and augmentor sections of gas turbine engines. The invention is particularly directed to nickel-base superalloy substrates that are prone to the formation of deleterious SRZ, and to a predominantly gamma-prime phase nickel aluminide (Ni3Al) overlay coating that reduces the incidence of SRZ when used as an environmental coating or as a bond coat for a thermal barrier coating (TBC) on the superalloy substrate. The gamma prime-phase nickel aluminide employed in the present invention is one of two stable intermetallic compounds of nickel and aluminum. The gamma prime-phase exists for NiAl compositions containing nickel and aluminum in an atomic ratio of about 3:1, while beta-phase nickel aluminide (NiAl) exists for NiAl compositions containing nickel and aluminum in an atomic ratio of about 1:1. Gamma prime-phase nickel aluminide has a nominal composition of, by weight, about 86.7% nickel and about 13.3% aluminum, in contrast to the beta phase with a nominal composition of, by weight, about 68.5% nickel and about 31.5% aluminum. Accordingly, the gamma prime-phase nickel aluminide overlay coatings of this invention are compositionally distinguishable from beta-phase NiAl overlay coatings, as well as diffusion aluminide coatings that are predominantly beta-phase NiAl.

[0009] According to a preferred aspect of the invention, an article is provided having a nickel-base superalloy substrate containing aluminum and at least one element that renders the substrate susceptible to the formation of a secondary reaction zone. A coating system lies on the superalloy substrate and includes a chromium-containing nickel aluminide intermetallic overlay coating of predominantly the gamma prime phase. The intermetallic overlay coating contains aluminum in an amount approximately equal to the aluminum content of the superalloy substrate so as to inhibit diffusion of aluminum from the overlay coating into the substrate.

[0010] In view of the above, the invention makes use of a superalloy substrate and coating combination in which the compositions of the substrate and coating are chemically similar in terms of aluminum content. As a result, there is a reduced tendency for aluminum to diffuse from the coating into the substrate, thereby reducing the likelihood that SRZ will form in the substrate. The combination of the superalloy substrate and overlay coating of this invention is particularly advantageous where the substrate is formed of a high strength nickel-base superalloy containing a significant amount of refractory elements, such as rhenium, tungsten, tantalum, hafnium, molybdenum, niobium, and zirconium, which are known to promote the formation of deleterious SRZ that contains TCP phases as a result of the diffusion of aluminum from an aluminum-containing coating.

[0011] In addition to the above, gamma prime-phase nickel aluminide intermetallic overlay coatings suitable for use with this invention are believed to have a number of additional advantages over existing overlay and diffusion coatings used as environmental coatings and bond coats for TBC. The gamma-prime phase (Ni3Al) is intrinsically stronger than the beta phase (NiAl), enabling the overlay coatings of this invention to better inhibit spallation events brought on by stress-related factors. Additional benefits are believed to be possible as a result of the higher solubility of reactive elements in the gamma-prime phase, such that much greater additions of these elements can be incorporated into the overlay coating to further improve the environmental resistance and strength of the coating.

Problems solved by technology

However, a thermal expansion mismatch exists between metallic bond coats, their alumina scale and the overlying ceramic TBC, and peeling stresses generated by this mismatch gradually increase over time to the point where TBC spallation can occur as a result of cracks that form at the interface between the bond coat and alumina scale or the interface between the alumina scale and TBC.

However, beyond the solubility limits of the reactive elements, precipitates of a Heusler phase (Ni2AlZr (Hf, Ti, Ta)) can occur that can drastically lower the oxidation resistance of the coating.

The oxidation and hot corrosion resistance of an environmental coating and the spallation resistance of a TBC deposited on a bond coat are complicated in part by the composition of the underlying superalloy and interdiffusion that occurs between the superalloy and the environmental coating or bond coat.

Because the boundary between SRZ constituents and the original substrate 22 is a high angle boundary that doesn't tolerate deformation, the SRZ 36 and its boundaries readily crack under stress, drastically reducing the load-carrying capability of the alloy.

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